The present invention pertains generally to processes for manufacturing polycrystalline graphitizable stock, and in particular to mixing procedures for the manufacture of polycrystallize graphitizable stock in which a fine-grain filler is used.
Graphitizable compositions usually contain a carbon particle filler such as calcined coke and a thermoplastic binder such as a coal tar pitch to bind the filler particles together. Instead of a thermoplastic binder, the binder may be thermosetting resin such as epoxy. Optional additives such as iron oxide may be included. Essentially a graphitizable composition is one which forms graphite upon application of sufficient heat.
It is most desirable to obtain a homogeneous distribution of binder and filler during the mixing cycle in the manufacture of poly-crystalline graphitizable articles. If not, agglomerated fine particles resist penetration of liquid binder and the result is either binder rich or binder lean areas within the molded or baked stock. Since the strength of graphite stock is to a great extent determined by the carbon bond formed by the binder between filler particles upon baking, such irregularities weaken the stock. Further these discontinuities allow for the entrapment of air during mixing which disrupts the microstructure during the baking. Thus, improvements in binder distribution around the filler particles would reduce part-to-part variations and improve the baked physical properties.
This problem is particularly serious with fine-grain (74 microns or 200-mesh-screen maximum particle size) carbon filler or fillers with a fine-grain component. Small particles have a high surface-area-to-volume ratio which denotes a large quantity of surface charges, both positive and negative. Accordingly, small particles tend to agglomerate to a greater extent than larger particles, and it is more difficult to obtain a uniform distribution of pitch and filler particles during mixing.
With present techniques, all ingredients for a given formulation are weighed and transferred to a mixer after any necessary preparation and sizing of the raw materials. In certain formulations the dry ingredients are often preblended in a twin shell blender or its equivalent, but this additional technique does not completely eliminate the agglomeration of the fine particles of the filler.